Motor Ratings

Continuous current, a specification which is also known as continuous ampere rating, measures the ability of motor starters and contactors to handle current over a continuous period.

Thermal current is directly proportional to the coefficient of thermal conductivity (k), the property of a material which indicates its ability to conduct heat.

Power ratings for motor starters and contactors vary by motor type. For example, AC motor starters and contactors carry ratings for single-phase power or three-phase power. DC motor starters have a DC horsepower rating.

Types

There are many different types of motor starters and contactors.

Multi-speed motor starters are designed to be operated at a constant frequency and voltage.

Reduced voltage starters (RVS) also change speeds, but use motor windings that can be reconnected to form different numbers of poles.

Reducing or reversing motor starters and contactors are designed for applications which must avoid overload conditions, or which need to avoid unnecessary wear and tear on equipment. There are five main types of reduced voltage starters: primary resistor, autotransformer, part winding, wye delta, and solid state.

Full voltage, non-reversing motor starters have an overload relay and two contactors. These motor starters and contactors are mechanically and electrically interlocked.

Manual motor starters are also commonly available. They connect the incoming power directly to the motor, and are well-suited for squirrel-cage motors.

Standards

Motor starters and contactors comply with ratings or standards from the National Electrical Manufacturers Association (NEMA) and the International Electrotechnical Commission (IEC).

There are 11 NEMA sizes: 00, 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. IEC sizes use letters instead of numbers, and are rated at either the thermal current rating or the operational current.